Radt-53. Impact of Smaller Clinical Target Volume Expansions for the Radiation Treatment of Newly Diagnosed Glioblastoma

BACKGROUND With advances in image-guidance, there has been interest in smaller clinical target volume (CTV) expansions for radiation therapy planning in glioblastoma. We reviewed the impact of varying CTV expansion margins on patterns of recurrence in two patient cohorts. METHODS 412 consecutive adult glioblastoma patients treated with chemoradiation at a single institution and were included in the analysis with available pre-radiation and first recurrence imaging. A separate prospectively collected imaging cohort of 22 patients (NCT04188535) was used for validation. Multiple CTV expansions (0-2cm) were applied to the pre-radiation segmentation of enhancing tumor(GTVgad) and T2/FLAIR abnormality(GTVflair). CTVs were cropped to brain. We focus on an approach generously incorporating T2/FLAIR abnormality (Approach 1: CTV1 = [GTVgad+1.5cm] + [GTVflair+1.5cm]) and a more focal approach prioritizing enhancing disease (Approach 2: CTV2 = [GTVgad+1.5cm] + [GTVflair]). RESULTS The retrospective cohort included 412 patients with a median age of 60.1 (range 21.0-94.0), 46% patients underwent gross total resection, 43% KPS> 90, and 39% were MGMT promoter methylated. On multivariable analysis, CTV volume was associated with OS(p<0.01). With a more comprehensive approach (Approach 1), CTV1 was median 269.9cc (range 56.2cc-907.7cc), and the epicenter of recurrent tumor was covered in 93% of cases. With Approach 2, the CTV2 was median 154.0cc (range 37.5cc-475.2cc), and the epicenter of recurrent tumor was covered in 90% of cases. Given comparable patterns of in-field failure with both approaches, we evaluated coverage of subsequent progressive disease in a validation cohort with the more focal approach (Approach 2), and CTV2 covered the epicenter of recurrent tumor in all 22 (100%) cases with a median CTV 175.4cc (range 71.9cc-316.2cc). CONCLUSION Smaller CTV margins significantly decrease integral brain radiation dose without a significantly increased risk of out-of-field failure. Target delineation approaches utilizing smaller CTV expansions in conjunction with advanced imaging may meaningfully decrease treatment-related toxicity.